The invention relates to a method for calibrating ultrasound clamp-on flowmeters using the transit time method.
Clamp-on flow-meters are widely used in many industrial fields. One of their essential advantages is that the flow measurement occurs in a contactless manner. With clamp-on flowmeter devices using the transit time difference method, the difference between two sound signals traveling with or against the direction of flow is measured, and the volume flow is calculated from this difference. The transit time difference is a function of the mean flow velocity VI on the sound path, the sound angle of incidence in the fluid, and the sound transit time tfl. The relationship is described by the following formula:VI=Ka*(Δt/2tfl)  Eq. (1)Ka is the acoustic calibration factor, which determines the angle of incidence in the fluid:Ka=cα/sin(α)  Eq. (2)
The angle of incidence in the fluid is expressed here using the law of refraction with angle of incidence and sound velocity in the forward portion of the sound transducer. In order to calculate the volume flow, the flow mechanics calibration factor KF must also be known. This calibration factor KF represents the ratio of the mean surface of the flow velocity to the mean flow velocity on the sound path:KF=VA/VI  Eq. (3)
Then, the volume flow Q with the cross-sectional area A of the pipe results inQ=KF*A*Ka*(Δt/2tfl)  Eq. (4)
The calculation of the acoustic calibration factor as the ratio of sound velocity and angle of incidence in the forward portion presupposes certain ideal assumptions. First, the sound transducers must be positioned optimally, that is, acoustically opposite one another. In practice there are always deviations from the optimum position. Moreover, the sound velocity in the forward portion of the sound transducer is a function of temperature. If the temperature of the measured object deviates significantly from the ambient temperature, the temperature in the sound transducer is also a function of location. Then the acoustic calibration factor can only be calculated numerically provided that the temperature distribution in the forward portion is known.
Also not taken into account by Ka are any effects of the pipe wall on the angle of incidence in the fluid.
Calibration is possible in cases in which the aforesaid deviations of the acoustic calibration factor become relevant and cannot be compensated by computation. Calibration is generally undertaken on a flow calibration stand. It is assumed that all of the variables contained in Eq. (4), i.e., volume flow Q, flow mechanics calibration factor KF, inner cross-sectional area of the pipe, and the transit time and transit time difference are known. The acoustic calibration factor can be calculated by reworking Eq. (4) to:Ka=Q/A*KF*(Δt/2tfl))  Eq. (5)
However, calibration stands are only available for a small number of the measurement conditions that occur in practice. Preparing a reference volume flow for wide nominal widths or high temperatures is very complex.